Special thanks to Thijs Terlouw and Sander Vermeulen for providing and helping to test this lens!

Introduction

The Sigma AF 10-20mm f/4-5.6 EX DC HSM is one of the several new players in the ultra-wide
zoom market. According to Sigma's naming conventions the lens is a designated DC (Digital Camera)
lens so it can only be used on reduced image circle (APS-C) DSLRs. On 35mm format cameras there'll
be unrecoverable edge shading through most of its zoom range so it's a no-no here. It's quite
interesting to note that Sigma has now 3 ultra-wide zoom lenses (from an APS-C perspective)
in their line-up. Besides the 10-20mm DC there's the 12-24mm f/4.5-5.6 EX as well as the
15-30mm f/3.5-4.5 EX - both full-frame lenses but naturally perfectly usable on APS-C DSLRs
as well. As of today the lens is available for Sigma, Nikon and Canon.
The field-of-view of the AF 10-20mm DC is equivalent to 16-32mm on a full frame camera.

The lens construction is made of 14 elements in 10 groups including 3 Special Low Dispersion
(SLD) elements and 3 aspherical elements (1 molded, 2 hybrid-type) - quite a bit of efforts here.
Its aperture mechanism features 6 aperture blades.
The lens is fairly compact with a size of 83x81mm and comparatively heavy regarding
its rather slow aperture range at 470g. Unlike its two ultra-wide sister lenses the lens does
not have a protruding front element - there's a standard (non-rotating) filter thread (77mm)
so using a polarizer is not a problem. A petal-type hood as well as a soft bag are part
of the package.

The lens has a very fast and near-silent HSM (hypersonic motor) AF drive which
provides full-time manual focusing in one-shot AF mode. The minimal focus distance is
0.24m resulting in a max. magnification of 1:6.7 at 20mm.
The build quality of the lens is very impressive - it feels like a solid block. The
typical EX finish (smooth crinkle style) feels very pleasant and the zoom
and focus control rings are very smooth and well damped. The lens does extend a little
towards the long end.